Efficacy of a nonviable culture-derived Babesia bovis vaccine.

Tissue culture-derived soluble Babesia bovis antigens were combined with a saponin adjuvant and used to vaccinate 3 adult cows and 8 yearling heifers. Each animal was vaccinated twice at a 3-week interval between vaccinations. Two adult cattle were premunized with an attenuated B bovis. All vaccinated cattle and premunized cattle along with 3 nonvaccinated adult cattle and 8 nonvaccinated yearling heifers were challenge exposed 4 weeks after the 2nd vaccine dose by IM inoculation of an estimated 1 x 10(8) virulent B bovis organisms. Both vaccinated and nonvaccinated controls responded to challenge exposure, whereas the premunized cattle were refractory to challenge exposure. These cattle given the adjuvant-vaccine showed a significantly milder clinical response to infection. Death occurred in 2 of the 3 adult controls and 2 of the 8 yearling controls. Death losses or severe reactions were not seen among the vaccinated cattle. Vaccinated animals, before challenge exposure, failed to respond serologically to the complement-fixation test even though specific antibodies to B bovis antigens were detected with the indirect antibodies to B bovis antigens were detected with the indirect fluorescent antibody test. The apparent absence of isoimmunizing antigens and the ability to serologically distinguish vaccinated animals from infected animals are additional attributes which contribute to the potential usefulness of this vaccine.